1. Field of the Invention
The present invention relates to a three-state discernment circuit, and more particularly to a circuit in which three states are discriminated by controlling one terminal.
2. Description of the Prior Art
In digital systems such as those used in electronic digital watches and the like, it is sometimes required to control three different states by an external terminal or terminals. For example, it is desired to control the brightness of display in a display device of an electronic digital watch in accordance with the conditions of light in the atmosphere in such a manner that a time for which the display device is excited is adjusted by a manual switch. Namely, since the luminous intensity in a discharge display device such as Digitron and Nixie or a light emitting display device such as LED (light emitting diode) is constant, the lower brightness in the display device gives some difficulty in observing a displayed pattern when the illumination in the atmosphere is high, i.e. when the atmosphere is light. Then, the brightness in the display device must be increased. In the night or when the atmosphere is dark, on the other hand, the display is sufficient in the low brightness and the higher brightness makes a displayed pattern obscure or makes the observation of the pattern difficult.
Therefore, it is desired to control three different states by means of an external terminal in such a manner that the brightness in a display device is established to a high level, a middle level and a low level.
A prior art digital system of such a kind includes a circuit in which a pair of controllable external terminals are provided for producing three states by the combination of 0 and 1. Another prior art digital system includes a circuit in which a single controllable external terminal is provided for producing such states by the combination of analog quantities of different levels, for example, a ground level, an open-circuit a level of -1/2 V.sub.B and a level of -V.sub.B (V.sub.B : power source voltage). However, the former prior art digital system has been defective in that four different combinations of 0 and 1 are produced by the provision of the two externally controlled terminals in the circuit resulting in the presence of the redundancy. The latter prior art digital system has also been defective in that a structure including such analog circuit portion in the digital system is difficult to be integrated into an LSI form, especially in the design of MOSFET's (for example, the control of the threshold voltage of MOSFET). Further, this latter prior art digital system has been defective in that the circuit operation tends to operate erroneously due to noises since the circuit is not adapted for synchronous operation.